It is known that a resin for a plastic lens having a high refractive index can be obtained by polymerizing a polyisocyanate compound and a polythiol compound. For example, PTL 1 discloses a method for producing a polyurethane plastic lens having a high refractive index by heating and curing a composition obtained by mixing an aliphatic polyisocyanate compound and an aliphatic polythiol compound such as pentaerythritol tetrakis(thioglycolate) and trimethylolpropane tris(thioglycolate).
Isocyanate compounds are unstable and react relatively easily with an active hydrogen compound due to a high reactivity demonstrated by the isocyanate group. Particularly problematic is the reaction of isocyanate compounds with moisture in the atmosphere. An isocyanate compound which has reacted with moisture in the atmosphere becomes cloudy or colored. For this reason, it is necessary to add a stabilizer in order to store the isocyanate compound. For example, PTL 2 discloses a plastic lens using a phenol as a stabilizer as a plastic lens which is required to be colorless, transparent and homogeneous, the lens being composed of an optical urethane resin having little coloration, high total light transmittance, and no optical distortion.